The present invention relates generally to fuel delivery systems for engines, especially gas turbine engines, and more particularly to such fuel delivery systems for aircraft such as helicopters and small fixed-wing turboprops incorporating engine temperature limiting features during engine start. The system reduces the pilot workload by automatically controlling the engine temperature during engine starts.
Many present control systems for turbine engines do not provide temperature limiting during engine start. Subsequently, under certain ambient conditions, battery conditions, manual control adjustments etc., the engine can overheat (xe2x80x9covertempxe2x80x9d) thus causing delay or cancellation of the flight or unscheduled engine maintenance. In addition, the pilot has full responsibility to cut off the fuel when a potential hot start may occur. For example, a low battery condition when attempting to start a gas turbine engine results in a slower than normal engine cranking speed; however, fuel is supplied at the normal rate resulting in an over-rich fuel/air mixture and increased engine heating during the start-up process. If a hot start occurs, the engine shut off and restart attempted, the battery may be in an even more depleted condition aggravating the problem.
An audible alarm indicative of over-stress limits including temperature, output torque and engine speed, instructing the pilot to provide the corrective action is shown in U.S. Pat. No. 4,619,110. Water and/or additional fuel may also be automatically injected or the engine fuel flow automatically reduced.
A proposed solution to automatically avoiding hot start problems is provided in U.S. Pat. No. 3,902,315 where fuel flow during engine start is initially controlled by an intentionally over-rich scheduled function of engine speed only. The fuel flow control is switched to an acceleration and speed control schedule upon sensing, inter alia, an excessive tailpipe temperature which temperature is a function of both compressor inlet pressure and engine speed.
Another U.S. Patent considering the hot start possibility is U.S. Pat. No. 4,350,008 where the fuel flow rate is reduced upon sensing an excessive turbine temperature and that reduced flow continued until the over-temperature condition is corrected.
U.S. Pat. No. 5,101,619 teaches a hot start correcting system where fuel flow to the engine is completely shut-off for a specific brief period of time when an overtemperature condition is sensed. Fuel flow is then resumed for a longer specific time period regardless of the sensed temperature.
All four of these patented schemes utilize one fuel control valve for all fuel flow control.
The hot start problem has also been addressed in U.S. Pat. No. 3,964,253 where an additional parallel fuel flow path provides enhanced fuel richness during start with that additional flow being shut-off in the event an excessive temperature is sensed.
None of these patented schemes continues to monitor temperature and tailor the reduced fuel flow to the monitored temperature.
It is desirable to provide automatic temperature limiting during engine start and to provide such temperature limiting in a continuous, economical, relatively simple and retrofitable way. An externally mounted fuel solenoid valve provides an inexpensive solution eliminating the need for additional calibration or recalibration of the main fuel control. Only external plumbing is required for field installation.
The present invention provides solutions to the above problems by providing a fuel burning engine overtemperature avoidance system which monitors an engine temperature indicative temperature such as exhaust gas temperature, and diminishes fuel flow to the engine, for example, by diverting a portion of the fuel flow to the engine from the main fuel control back to the engine fuel pump, when the monitored temperature exceeds a threshold. A pulse width modulated control signal to a solenoid operable valve cyclically enables and disables the return flow of a portion of the fuel flow to the engine from the main fuel control back to the engine fuel pump. The period of cyclic enabling and disabling is fixed and the fraction of each cycle during which enabling occurs is controlled in accordance with the monitored exhaust gas temperature. Undiminished fuel flow is resumed when the monitored temperature falls below the threshold temperature. An engine operating parameter such as gas generator speed, is also monitored and fuel flow diminution to the engine is prevented despite the monitored temperature exceeding the threshold temperature when the monitored parameter exceeds a predetermined level such as engine idle speed. The threshold temperature may be determined by measuring ambient temperature and computing the threshold as a prescribed increment above the measured ambient temperature. Recording monitored temperature, gas generator speed and fuel flow diminution, allows subsequent modification of the engine fuel system to provide enhanced engine performance achieving overtemperature avoidance with reduced fuel flow diminution.